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Epizootic and Enzootic Aspects of Minchinia nelsoni (Haplosporida) Disease in Maryland Oysters
Author(s) -
FARLEY C. AUSTIN
Publication year - 1975
Publication title -
the journal of protozoology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.067
H-Index - 77
eISSN - 1550-7408
pISSN - 0022-3921
DOI - 10.1111/j.1550-7408.1975.tb05197.x
Subject(s) - epizootic , enzootic , biology , population , oyster , crassostrea , bay , incidence (geometry) , fishery , zoology , veterinary medicine , demography , outbreak , geography , medicine , virus , physics , archaeology , virology , sociology , optics
SYNOPSIS.Minchinia nelsoni disease in oysters ( Crassostrea virginica ) from Marumsco Bar, Pocomoke Sound, Maryland (an estuarine tributary of Chesapeake Bay) was studied for 8 years (1961–68) to determine epizootiologic relationships concerning life cycle of the parasite, pathologic effects on the host, and effects of physical factors on population density and recruitment of the host and parasite. The study period covered pre‐epizootic, epizootic, and post‐epizootic disease conditions. Data on the native oyster population as well as annual introductions of previously unexposed, susceptible populations of juvenile oysters from 1965–68 were included. Salinity, water temperature, mortality, prevalence, incidence, life cycle stages, gross pathology, and histopathologic relationships were observed. Mortality was high (45–55% per year) during the first 3 years of the study; however, M. nelsoni prevalences were low (> 25%) and did not clearly imply a cause and effect relationship. Drought conditions that began in the summer of 1963 and continued through 1967 caused higher salinities, and apparently initiated epizootic disease in the native oyster population. The epizootic peaked in May 1965 with a diagnosed prevalence in native oysters of 70%. Enzootic levels of annual mortality (40% in 1966, 30% in 1967, and 2% in 1968) and fall prevalence (16%, 24%, and 4%) developed after that time. Introduced populations had a typical epizootiologic pattern in 1965 (55% annual mortality, 82% incidence) and 1966 (55% annual mortality, 66% incidence) which declined in 1967 (30% annual mortality, 44% incidence) followed by a disappearance of the disease in 1968. Epizootiologic differences noted between native oysters (adult and juvenile) and the introduced juvenile populations were also evident from the stages of the disease. Infections in native animals tended to be less serious, and in many cases were delayed or attenuated, while infections in introduced oysters progressed to advanced or terminal phases. Occult manifestations (mantle recession thought to be due to M. nelsoni in oysters not showing histologic evidence of infection) were absent in introduced populations and common in the native population. These differences are interpreted as evidence of resistance in surviving native oysters and their progeny, and may indicate genetic resistance developed by natural selection and manifested by an increased ability to survive and overcome infection.